A control device of this type is known (German patent letter No. 2 950 881). The spindle is rotatably supported in a paraxial sleeve, which in its turn is supported in the bearing member for the spindle. The sleeve is adapted to be elastically deformed in at least one region and carries an elongation measuring strip the signals of which are compared with preset signals in a comparison device. The actual signals indicate the respectively prevailing processing pressure. The bearing member for the spindle is designed as a double-acting hydraulic piston which is displaceably supported in a cylinder fastened to the frame.
The known device suffers from some disadvantages.
The thrust sensor, it is true, is in a position to provide signals exactly in proportion to the processing pressure, however, it is very expensive in construction. Furthermore, the thrust sensor is not readily accessible and, therefore, the mounting thereof necessarily is very expensive and, in case repair work has to be carried out, also the demounting thereof involves considerable costs. Another disadvantage resides in that the adjustment of the tool is effected via a hydraulic drive. A hydraulic drive likewise is relatively expensive. Furthermore, a hydraulic drive constitutes a rigid system which in its effectiveness shows a relatively stiff charateristic.
So as to avoid undesired impairments of the workpiece surface, efforts are made in connection with machines for precise surface processing not to have the processing pressure build up suddenly, but gradually, while avoiding a too hard engagement; only after a certain time the tool, such as a lapping disc, for example, is to come to lie onto the workpiece with the rated pressure. On the other hand, the build-up of the processing pressure is to take place as quickly as possible, in order to reduce the production time. However, it is relatively difficult to obtain soft transitions in an attempt to avoid damage to the workpieces, especially if non-linear and non-uniform load curvatures, respectively, are to be followed. Owing to frictional resistances in the hydraulic drive and a characteristic hysteresis it causes problems with each new processing step to obtain an exactly reproducible pressure build-up. Only the latter ensures an optimum surface processing of the individual workpieces.